An electronic control unit (ECU) electronically controlling an engine or a transmission is equipped with a power supply control device that uses an onboard battery voltage as an input voltage from the outside, adjusts the battery voltage to a predetermined voltage, and supplies appropriate voltage and current to various power supply targets. Examples of the power supply target include a microcontroller or various integrated circuits (ICs) mounted in the ECU and various sensors connected to the outside of the ECU. Since voltages to be supplied to the power supply targets are generally lower than the onboard battery voltage, the power supply control device steps down the onboard battery voltage to voltages suitable for input voltages of the power supply targets.
Recently, vehicles equipped with an idling stop system that stops idling of an engine when a vehicle stops as measures for improvement in fuel efficiency have increased more and more. It is necessary to drive a starter when the engine is restarted from an idling stop state, but the driving of the starter requires supply of power from a battery and thus a temporary decrease in battery voltage. Accordingly, since the ECU more frequently requires an operation at a low battery voltage, it is necessary to guarantee a satisfactory operation at a low battery voltage. There is demand for a power supply control device that can maintain supply of appropriate voltage and current to a power supply target even at a low battery voltage.
Conventionally, a power supply control device including a step-down switching regulator and a series regulator in consideration of power conversion efficiency and output voltage ripples is known as such a type of power supply control device (for example, see PTL 1). In general, the step-down switching regulator enables power conversion at higher efficiency than that of the series regulator, but the output voltage ripple is larger than that of the series regulator, which may cause a problem, for example, when the step-down switching regulator is used for a reference voltage of an analog-to-digital (AD) conversion circuit. Accordingly, by converting the battery voltage into a predetermined voltage as an intermediate voltage at high efficiency using the step-down switching regulator and stepping down the intermediate voltage to a voltage suitable for the power supply target using the series regulator, the power conversion efficiency and the output voltage ripple of the power supply control device are made to be compatible with each other.
FIG. 17 is a diagram illustrating a configuration of an electronic control device according to a conventional example.
A power supply control device 4 includes a first power supply 1, a second power supply 2, and a third power supply 3.
A battery voltage 41 is input as an input voltage to the power supply control device 4, and the battery voltage 41 is input to the first power supply 1 via a reverse connection prevention diode 42.
The first power supply 1 is a step-down switching regulator and steps down a first power supply input voltage 44 to a first power supply output voltage 17. The first power supply 1 includes a switching element 11, a freewheel diode 15, an inductor 14, and a first voltage control circuit 12. When the first voltage control circuit 12 instructs turning-on, the switching element 11 supplies the first power supply input voltage 44 to the inductor 14 and supplies a current to the rear stage of the first power supply 1. On the other hand, when the first voltage control circuit 12 instructs turning-off, the switching element 11 does not supply the first power supply input voltage 44 to the inductor 14 side and supplies a current to the rear stage of the first power supply 1 by discharging energy stored in the inductor 14 via the freewheel diode 15. In this way, a switching output voltage 13 is the first power supply input voltage 44 when the first voltage control circuit 12 instructs turning-on, and is a reference potential 45 when the first voltage control circuit 12 instructs turning-off. The first voltage control circuit 12 monitors the first power supply output voltage 17 and controls the switching element 11 in a pulse width modulation (PWM) manner such that the first power supply output voltage 17 is a predetermined voltage.
The second power supply 2 is a series regulator that supplies a voltage to a microcontroller 5. The second power supply 2 includes a second power supply output transistor 21 and a second voltage control circuit 22. The second voltage control circuit 22 monitors a second power supply output voltage 24 and controls the second power supply output transistor 21 using the first power supply output voltage 17 as an input voltage such that the second power supply output voltage 24 is a predetermined voltage.
The third power supply 3 is a series regulator that supplies a voltage to, for example, a sensor outside the electronic control device, other than the microcontroller 5. The third power supply 3 includes a third power supply output transistor 31 and a third voltage control circuit 32. The third voltage control circuit 32 monitors a third power supply output voltage 34 and controls the third power supply output transistor 31 using the first power supply output voltage 17 as an input voltage such that the third power supply output voltage 34 is a predetermined voltage. In the following description, it is assumed that the third power supply output voltage 34 is controlled to the same voltage as the second power supply output voltage 24.
The power supply control device 4 includes a voltage generating function control register 36 for the third power supply 3. When a third power supply output-ON control signal 110a is transmitted to the voltage generating function control register 36 by serial communication or the like, the voltage generating function control register 36 becomes high, the third power supply 3 is turned on, and the third voltage control circuit 32 monitors the third power supply output voltage 34 and controls the third power supply output transistor 31 such that the third power supply output voltage 34 is a predetermined voltage. On the other hand, when a third power supply output-OFF control signal 110b is transmitted to the voltage generating function control register 36 by serial communication or the like, the voltage generating function control register 36 becomes low, the third power supply 3 is turned off, and thus the third power supply output transistor 31 is turned off to stop the supply of power as a power supply.
The microcontroller 5 generally has a guaranteed operating range for a source voltage, and when a source voltage outside the guaranteed operating range is supplied, the operation of the microcontroller 5 is not guaranteed. Accordingly, when the source voltage of the microcontroller 5 is outside the guaranteed operating range, a reset signal 71 is output to the microcontroller 5 to prevent an unexpected operation of the microcontroller 5. In order to generate the reset signal 71 using the power supply control device 4, the power supply control device 4 includes a second power supply low output voltage detection circuit 25 for the second power supply output voltage 24. The second power supply low output voltage detection circuit 25 detects a low voltage of the second power supply output voltage 24 and outputs a second power supply low output voltage detection output signal 72, and a reset signal generation circuit 71a generates the reset signal 71 and outputs the reset signal 71 to the microcontroller 5 when the second power supply output voltage 24 is continuously in the low-voltage state.
A suppliable current value, that is, a current capacity, is set in circuit configuration for each of the first power supply 1, the second power supply 2, and the third power supply 3. When a current larger than the current capacity is drawn out from the power supply output, voltage control of stepping down a voltage to a predetermined voltage is not possible and a voltage value lower than a target voltage value is acquired. Particularly, since the third power supply 3 supplies a voltage to an ECU-outside sensor, there is a possibility that a signal line of the third power supply output voltage 34 will be grounded. In this case, a third power supply output current is equal to or larger than the current capacity, which causes the above-mentioned phenomenon.
As described above, the second power supply 2 and the third power supply 3 are regulators connected to the rear stage of the first power supply 1. Accordingly, a first power supply output current is the total sum of the second power supply output current and the third power supply output current.
Now, operations of the power supplies at a low battery voltage at which the battery voltage 41 is low and the first power supply input voltage 44 is equal to or lower than a step-down control voltage value of the first power supply 1 will be described.
The first power supply 1 cannot control the first power supply output voltage 17 to a predetermined voltage value due to an insufficient input voltage on the basis of characteristics of the step-down switching regulator. Since the first power supply output voltage 17 is equal to or lower than the step-down control voltage value of the first power supply 1, the switching element 11 is controlled to be fully turned on to increase the first power supply output voltage 17. At this time, the first power supply output voltage 17 is a voltage which is obtained by subtracting an ON-resistance value of the switching element 11, a series resistance value of the inductor 14, and a voltage drop determined by the first power supply output current value from the first power supply input voltage 44.
The second power supply 2 uses the first power supply output voltage 17 which is lower than a normal voltage as an input voltage and controls the second power supply output voltage 24 to a predetermined voltage. In the series regulator, a minimum potential difference (a dropout voltage) between an input and an output is set on the basis of the characteristics of the output transistor. Accordingly, the second power supply output voltage 24 is controlled to a target voltage value when a difference between the first power supply output voltage 17 and a control voltage value of the second power supply 2 is equal to or higher than the dropout voltage, but becomes a voltage obtained by subtracting the dropout voltage from the first power supply output voltage 17 due to an insufficient input voltage when the difference between the first power supply output voltage 17 and the control voltage value of the second power supply 2.
The third power supply 3 exhibits the same behavior as the second power supply 2, and the third power supply output voltage 34 is controlled to a target voltage value when a difference between the first power supply output voltage 17 and a control voltage value of the third power supply 3 is equal to or higher than the dropout voltage, but becomes a voltage obtained by subtracting the dropout voltage from the first power supply output voltage 17 due to an insufficient input voltage when the difference between the first power supply output voltage 17 and the control voltage value of the third power supply 3.
When the guaranteed operating range of the battery voltage of the electronic control device includes the above-mentioned low battery voltage, it is necessary to set the ON-resistance value of the switching element 11 of the first power supply 1, the series resistance value of the inductor 14, the dropout voltage of the second power supply 2, and the dropout voltage of the third power supply 3 in consideration of the above-mentioned details and current consumption of a power supply target at the time of design. Here, the ON-resistance value of the switching element 11 of the first power supply 1, the dropout voltage of the second power supply 2, and the dropout voltage of the third power supply 3 greatly depend on the areas of the output transistors used in the power supplies. Specifically, in order to decrease the ON-resistance value of the switching element 11 of the first power supply 1, it is necessary to increase the area of the output transistor used in the switching element 11. In order to the dropout voltages of the second power supply 2 and the third power supply 3, it is necessary to increase the areas of the output transistors of the second power supply 2 and the third power supply 3.
As described above, since the third power supply 3 supplies a voltage to a sensor outside the electronic control device, there is a possibility that the signal line of the third power supply output voltage 34 will be grounded. When this phenomenon occurs, the third power supply output current becomes larger than the current consumption of the power supply target and increases up to the current capacity of the third power supply 3 in maximum. The increase in the third power supply output current is an increase in the first power supply output current.
A case in which a ground failure of the third power supply output voltage 34 occurs at a low battery voltage will be described below with reference to FIG. 18.
When a ground failure occurs in the third power supply output voltage 34, a first power supply output current 66 increases with an increase in a third power supply output current 68. The first power supply 1 controls the switching element 11 to be fully turned on at a low battery voltage. Accordingly, when the first power supply output voltage 17 decreases with an increase in the first power supply output current 66, the input voltage of the second power supply 2 is insufficient and the second power supply output voltage 24 cannot be controlled with a control voltage 61 for the second power supply. Until the second power supply output voltage 24 is stabilized to a voltage obtained by subtracting the dropout voltage from the first power supply output voltage 17, electric charges accumulated in a second power supply output capacitor 23 supplies current consumption of the microcontroller 5 which is a supply target of the second power supply 2.
In this way, when the second power supply output voltage 24 decreases and is lower than a second power supply low output voltage detection threshold 64, the second power supply low output voltage detection output signal 72 is generated and the reset signal 71 is output to the microcontroller 5 after a reset signal generation filtering time 75. Accordingly, when a ground failure of the third power supply output voltage 34 occurs at a low battery voltage, the battery voltage is within the guaranteed operating range of the electronic control device but the power supply control device 4 stops the operating of the microcontroller 5 and thus there is a problem in that the electronic control device cannot function normally.
In order to avoid the above-mentioned problem, a method of decreasing the ON-resistance value of the switching element 11 of the first power supply 1 and the dropout voltage of the second power supply 2 so as for the power supply control device to control the second power supply output voltage 24 to a target voltage value is used in consideration of a case in which the ground failure of the third power supply output voltage 34 occurs at a low battery voltage. However, this method causes an increase in area of the output transistor which is used in the power supply control device as described above, thereby causing an increase in cost of the power supply control device. In consideration of a state in which the ground failure of the third power supply output voltage 34 does not occur, specifications having excessive characteristics are obtained, thereby interfering with optimization of the function and the cost.
A problem in a case in which the battery voltage 41 is disconnected while a ground failure occurs in the third power supply output voltage 34 will be described below with reference to FIGS. 19 and 20.
FIG. 19 is a timing chart in a case in which the battery voltage 41 is disconnected when a ground failure does not occur in the third power supply output voltage 34.
When the battery voltage 41 is disconnected, a power supply control device input capacitor 43 functions as a battery of the power supply control device and the power supply control device operates, but electric charges accumulated in the power supply control device input capacitor 43 decrease with the operation of the power supply control device and thus the first power supply input voltage 44 gradually decreases. When the second power supply output voltage 24 also decreases with the decrease in the first power supply input voltage 44 and becomes lower than the second power supply low output voltage detection threshold 64, the second power supply low output voltage detection output signal 72 is generated and the reset signal 71 is output to the microcontroller 5 after the reset signal generation filtering time 75.
When a ground failure does not occur in the third power supply output voltage 34 and the second power supply output voltage 24 is higher than a microcontroller guaranteed operating voltage range lower limit 62, the reset signal 71 is output and the operation of the microcontroller 5 is limited to an operation within the guaranteed operating range for the source voltage of the microcontroller 5. Accordingly, the microcontroller 5 operates as designed.
FIG. 20 is a timing chart in a case in which the battery voltage 41 is disconnected when a ground failure occurs in the third power supply output voltage 34.
When a ground failure occurs in the third power supply output voltage 34, the first power supply output current 66 increases with the increase in the third power supply output current 68. With the increase in the first power supply output current 66, a decreasing speed of the first power supply input voltage 44 after the battery voltage 41 is disconnected is higher than that when a ground failure does not occur in the third power supply output voltage 34 and the decreasing speed of the second power supply output voltage 24 also becomes higher. Accordingly, when the second power supply output voltage 24 is lower than the microcontroller guaranteed operating voltage range lower limit 62, there is a possibility that the reset signal 71 will be output. That is, there is a possibility that the operation of the microcontroller 5 is not limited to an operation within the guaranteed operating range for the source voltage of the microcontroller 5 and thus the microcontroller 5 performs an unexpected operation.